Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor.
One common issue in image sensors, either of the CMOS or CCD type, is channel hot carrier (CHC) reliability. In order to resolve this issue, a heat treatment, such as an alloying process has been developed in the prior art. The purpose of the allowing process is to satisfy any “dangling” silicon bonds at the silicon/silicon dioxide interface underneath transistor gates. Hydrogen atoms are formed as part of the alloy process and these hydrogen atoms satisfy the dangling silicon bond and eliminate surface state traps and improve transistor performance.
During this alloying process, the semiconductor wafers are typically exposed to a combination of nitrogen (N2) and hydrogen (H2) gases in a furnace at a temperature of around 400-450 C. This process is performed as part of a passivation process or after etching of the bond pads of the image sensor. However, at this juncture of the manufacturing process, the metal layers of the image sensor have already been defined and patterned. The metal layers are typically detrimentally affected by temperatures greater than 400-450° C.